B

back e.m.f. - e.m.f generated that opposes the normal flow of current, as in a motor
background radiation - radiation around you all the time and not due to the source under test
band spectrum - spectrum composed of bands of colour – produced by molecules
bandwidth - the range of frequencies in a group of electromagnetic waves
bar - unit of pressure (1 bar = 10⁵ Pascal, 1 millibar = 100 Pascal)
bar breaker - apparatus for showing forces in thermal expansion and contraction (peg breaker)
bar code - laser read identification mark placed on many things we buy
bar magnet - magnet in the form of a straight bar (rectangular cross section)
barlow lens - concave lens used to increase the effective focal length of a telescope objective
barometer - instrument for measuring atmospheric pressure
baryon - subatomic particle heavier than a proton and made of three quarks
Beaufort scale - the scale of wind speed
beats - changing sound formed by two 'overlapping' sounds of similar frequency
becquerel - unit of radioactive decay. One disintegration per second. Symbol Bq.
Beehive cluster - open cluster of stars in Cancer (M44, Praesepe)
bel - sound level is one bel higher if the intensity of one source is ten times the other
bell jar - bell shaped glass jar used in air pressure experiments (with vacuum pump)
bends - painful gas expansion in blood vessels of divers rising too quickly from deep water
bernoulli effect - change of pressure in a moving fluid. High speed low pressure, as in wings.
beta particle - a charged particle emitted in some radioactive decays. An electron.
Big Bang - the giant explosion at the 'beginning' of the Universe
binding energy - energy needed to separate the nucleons (protons and neutrons) in a nucleus
bimetallic strip - a strip made of two different metals fixed together. Bends when heated.
binary - number expressed in ones and zeros. Base 2. Decimal 6 in binary is 110.
boiling point - the temperature where visible evaporation (bubbling) occurs throughout the liquid
binary star - one of a pair of stars that are orbiting each other (see optical binary)
bistable - a circuit that has two stable states
boiling water reactor - a nuclear fission reactor where water is used as both moderator and coolant
black body - the most efficient radiator of heat energy at a given temperature
black dwarf - final stage in the life of a normal star; a small, black, cold remnant of the star
black hole - astronomical object with an escape speed greater than the speed of light
blind spot - the point on the retina where the optic nerve joins. No vision cells here.
blue moon - if there are two full moons in any calendar month the second is called a Blue Moon
blooming of lenses – coating the lens with a thin film to reduce internal reflection
Bode's law - a system of numbers that gives the approximate distances of planets from the Sun
bolometer - instrument for receiving the total radiant energy from a body
bourdon gauge - apparatus for measuring gas pressure using a coiled metal tube
boson - particle having integral spin
boss - device on a clamp (retort) stand that holds the clamp
Boyle's law - for a gas pressure x volume is constant at a constant temperature
breakdown - a sudden electrical discharge between two objects
bremsstrahlung - braking radiation, radiation emitted by the sudden deceleration of an electron
brittle - a material that will shatter if subjected to a large stress e.g. glass
bubble chamber - apparatus for observing the track of a subatomic particle as a steam of bubbles
buoyancy - upthrust on a floating object
broadband - method of gaining faster and more reliable access to the Internet
brownian motion - random motion of small particles in a liquid (or gas) due to molecular impacts
brush - contact that allows current to enter and leave an electric motor or generator